Units code

There are a fixed set of specific units codes which are recognised by units conversion routines.

All instrument types

• '$t' - time-of-flight (microseconds)
• '$v' - neutron speed (m/s)
• '$tau' - inverse speed (s/m)
• '$lam' - wavelength (Angstrom)
• '$k' - wavevector (Angstrom^-1)
• '$e' - energy (meV)
• '$q' - momentum transfer (Angstrom^-1)
• '$sq' - momentum transfer squared (Angstrom^2)

Note: In the case of inelastic instruments, the units code is intelligently interpreted in units conversion routines to refer to incident or final neutrons according to whether the instrument is indirect or direct geometry. For example 'e' will refer to the final energy if direct geometry, initial energy if indirect geometry.

Elastic instruments only

• '$d' - d-spacing (Angstrom)

Inelastic instruments only

• '$w' - energy transfer (meV)
• '$wn' - energy transfer (cm^-1)
• '$thz' - energy transfer (THz)
• '$q+' - momentum transfer (+ve energy transfer)
• '$q-' - momentum transfer (-ve energy transfer)
• '$sq+' - momentum transfer squared (+ve energy transfer)
• '$sq-' - momentum transfer squared (-ve energy transfer)

Note: For inelastic instruments a given momentum transfer corresponds to two times-of-flight or energy transfers. The units type 'q+' and 'q-' enable the two branches of solutions to be separately selected.

Although the units conversion routines intelligently intepret the codes 'v', 'tau', 'lam', 'k' and 'e' to refer to the final or incident neutrons depending on the instrument type being direct or indirect geometry, the code can be explicitly set as below.

Direct geometry only

• '$v2' - final neutron speed (m/s)
• '$tau2' - final inverse neutron speed (s/m)
• '$lam2' - final wavelength (Angstrom)
• '$k2' - final wavevector (Angstrom^-1)
• '$e2' - final energy (meV)

Indirect geometry only:

• '$v1' - final neutron speed (m/s)
• '$tau1' - final inverse neutron speed (s/m)
• '$lam1' - final wavelength (Angstrom)
• '$k1' - final wavevector (Angstrom^-1)
• '$e1' - incident energy (meV)